Dim headlights on one side I've had like that before were a problem at the headlight connector. Any initial poor contact results in heat, promoting more oxidatioan, weakening of the thin contact fingers and ultimately deforming the pin holder connector. Peel back the boot and try pushing the wires in with the headlights on to see if things change. I know some have had to resort to cutting them off and splicing in a new connector. I seem to have caught mine early enough that I was able to clean things up, use a pin to pry the connector pins in for a snugger fit and finish by pushing the contact wires in from behind, making extra slack in the wires inside the boot so they don't want to pull out again. It took a couple of tries to get my cleanup to last more than a few months, the most recent effort has now lasted well over 5 years still with the original and slightly damaged connector.

The headlight switch making the wipers work of course smells like a short rather than a component or switch failure as there are few common devices and connections. Wiring shorts aren't common, but may happen where wires are badly pinched somewhere between metal. I'm thinking more along the line here of a poor ground connection in that headlight circuit that is resulting in voltage backfeeding into the wiper circuit in order to reach chassis ground, so I'd start by getting that headlight bright again. Backfeeding into the wiper circuit might also be caused by a poor contact at one of the headlight relays. A known problem area for high draw currents is the plastic sockets in the relay tray. After the headlamp connector is verified as okay, I'd next check under the tray, paying particular attention to the sockets and wires from the headlight relay (front row K), headlight flasher relay (2nd row, 3rd socket from left) and wiper relay (back row, 2nd socket from right). After visual inspections, it would be time to put a meter to work doing fault tracing on isolated circuits. The bulb out sensor relay is of course another common problem area. If you've got a spare or made up a bypass out of a bad one (as I keep on hand) then maybe try that before going to the effort of pulling out and flipping the relay tray.

I just saw Bulletproof's thoughts about broken tailgate wiring. 240's were definitely prone to this, but not 700/900 wagons with a different tailgate harness design. The exception would be a wire inside the tailgate chafing against a wiper linkage arm, but there are no headlight/park light circuits in the tailgate to short.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now